By way of example, such a measurement apparatus is described in WO 2006/077145 A2. This measurement apparatus comprises a Fizeau interferometer, with which a measurement wave with a wavefront that is adapted to the intended shape of the optical surface is produced. The wavefront of the adapted measurement wave is evaluated by interferometry after reflection of the optical surface for the purposes of determining the deviation of the actual shape of the optical surface from the intended shape thereof. The Fizeau interferometer according to WO 2006/077145 A2 is a so-called common path interferometer, in which the paths of the interfering partial beams extend along the same optical path or are at least tightly adjacent to one another in space. However, such common path interferometers are susceptible to phase disturbances that are caused by mechanical vibrations of the test object or by air schlieren in the interferometer cavity. This phase susceptibility can be prevented by the use of a multi-fringe interferometer. In such a multi-fringe or multiple fringe interferometer, the interfering partial beams extend with such a tilt in relation to one another that a multi-fringe interference pattern arises.
By way of example, the optical element with the optical surface is an optical component, such as, for instance, a lens element or a mirror. Such optical components are used in optical systems, for instance in a telescope used in astronomy or in an imaging system, as is used in lithographic methods. The success of such an optical system is substantially determined by accuracy with which the optical components thereof can be produced and processed to the effect of the surfaces thereof corresponding to an intended form in each case, said form being set by a designer of the optical system during the design thereof. Within the scope of such production, it is necessary to compare the form of the processed optical surfaces with the intended form thereof and determine differences or deviations between the manufactured surface and the intended surface. Then, the optical surface can be processed in those regions in which the differences between the processed face and the intended face exceed predetermined thresholds, for example.
Disturbances that can be traced back to defects on the optical surfaces of the interferometer often occur during the highly precise measurement of optical surfaces using multi-fringe interferometry. Here, the illumination in these interferometers can be chosen in such a way that the disturbances are minimized to the best possible extent. This can be implemented by virtue of using an extended circular light source in the illumination pupil instead of a “punctiform” light source with an extent of no more than one airy diameter. The extended light source leads to unsharp imaging of the defect and hence to a significant reduction in the measurement error. However, a disadvantage thereof is that the contrast of the multi-fringe interferogram is reduced, leading to a poorer signal-to-noise ratio.